The field of this invention relates to slurries of abrasive inorganic oxide particles and methods for altering and controlling the abrasiveness of the particles for use in cleaning and polishing applications. In particular, it relates to altering or controlling the abrasivity of inorganic oxide particles used in chemical/mechanical polishing (CMP) processes carried out to planarize electronic chips.
The function of an abrasive slurry in chemical/mechanical polishing electronic chips is to polish or planarize either an insulating or conducting layer deposited on the chip to a highly planar state. This planarization maximizes sharpness of focus in subsequent photo-lithography steps used to deposit additional insulating and conductive layers. The slurry also must provide uniform polishing across the wafer without undue scratching or pitting of the polished substrate. While meeting these requirements it is also desirable to maximize polish rate in order to maximize the productivity of high-cost polishing equipment.
It therefore would be desirable to improve present slurries and their methods of manufacture so that one can easily modify the slurry to produce particles having a range of abrasivities. Having such a method would allow one to optimize the polishing process to yield the highest polish rate without undue scratching, non-uniformity of polishing or loss of process control. Such a method would be especially useful in developing new abrasive slurries for insulating and conducting materials being considered for use in chip manufacturing in the near future.
In general, conventional slurries comprise abrasive particles and/or soluble chemical ingredients. The particles and additional ingredients alter the abrasive and/or polishing effects imparted by the slurry. The particles and ingredients are selected to meet the polishing requirements for the insulating or conducting layer being polished.
U.S. Pat. No. 5,527,423 to Neville, et al. is an example of such slurries. It discloses CMP slurries comprising fumed silicas or fumed alumina particles dispersed in a stable aqueous medium. Neville also mentions that precipitated alumina can be used. Neville et al. disclose that the particles have a surface area ranging from about 40 m2/g to about 430 m2/g, an aggregate size distribution less than about 1.0 micron and a mean aggregate diameter less than about 0.4 micron. This patent also discusses references that teach the addition of etchants, such as hydrogen peroxide, or alkaline materials to CMP slurries. Other patents that disclose CMP slurries containing hydrogen peroxide and/or other acidic or alkaline additives include U.S. Pat. Nos. 5,700,838 to Feller, et al., 5,769,689 to Cossaboon, et al., 5,800,577 to Kidd and 3,527,028 to Oswald. In general, slurries such as these are based on the concept of selecting an inorganic oxide particle and either relying on the particles"" inherent abrasive properties for polishing or by adding additional chemistry to the slurry in order to adjust the abrasive and/or polishing effects imparted by the slurry.
U.S. Pat. No. 4,304,575 to Payne discloses the preparation of aqueous silica sols for use as abrasive materials in mechanically polishing semiconductor wafers. Payne""s method for preparing the sol comprises heating an initial alkaline aqueous silica sol containing a mixture of relatively smaller particles and relatively larger particles. It is stated by Payne that the smaller particles dissolve and redeposit on larger particles thereby producing an aqueous silica sol in which the majority of the silica particles have a size significantly larger than the larger silica particles in the starting mixed sol. Payne""s materials are prepared from sols having average particle size less than 100 millimicrons and preferably having final particle size of about 180 millimicrons. A similar disclosure is set forth in U.S. Pat. No. 4,356,107 also to Payne.
It is still desirable to devise methods of making abrasive slurries in such a way that the abrasiveness of the particles can be easily adjusted to meet the polishing requirements at hand without having to resort to additional chemistry or a new starting material for the abrasive particle.
In this invention, aqueous slurries of fine, porous, inorganic oxide particles are heated, e.g., in an autoclave, to increase the particles"" abrasivity. These slurries are preferably prepared directly from slurries having a median particle size in the range of 0.1 to about 0.5 micron and wherein substantially all of the particle size distribution is below one micron. Slurries produced by this process have abrasive properties such that an alkaline slurry (e.g., at pH 10.8) consisting of water and the inorganic oxide particles removes silica at a rate of at least 120 mm/ minute at 200 psixc2x7rpm. This measurement was made at a solids content of 12.6% by weight, at a pH of about 10.8 and with a Strasbaugh 6CA polisher with a SUBA 500 pad at a two minute polish time.
As mentioned above, autoclaving slurries of fine particle, porous, amorphous silica imparts an increased abrasiveness to the particles. This is reflected in increased removal rates of silica substrate at standard polishing conditions. This increase in particle abrasivity strongly correlates with a decrease in particle surface area as determined by N2 adsorption (BET method). It is thought that this increase in particle abrasiveness and associated decrease in particle surface area is attributable to silica transport during the autoclaving process whereby silica is preferentially dissolved from sharply convex surfaces within the porous particle and redeposited at sharply concave surfaces at the junction of silica subunits (ultimate particles) that make up the porous particle. This redeposition should thus strengthen the porous silica particle and increase its abrasivity. The elevated temperatures associated with autoclaving serve to accelerate this dissolution-redeposition process by increasing silica solubility. A similar process takes place in alkaline aqueous suspensions of silica particles held at room temperature or temperatures up to ambient pressure boiling (xcx9c100xc2x0 C. ), but much longer times would be required.
Accordingly, a method for imparting a desired abrasivity for a slurry of particles can be devised by thermally treating the slurries to predetermined surface areas which have been identified with a particular polishing rate. The method comprises
(a) preparing a slurry of porous inorganic oxide particles having measurable BET surface area,
(b) heating the slurry to obtain a particle BET surface area previously identified to have the desired abrasivity as measured by a polishing rate, and
(c) adjusting the slurry to be suitable for use as an abrasive slurry.
This invention is especially suitable for preparing slurries of silica gel particles. Fumed and precipitated silica slurries also can be similarly processed to undergo an increase in abrasivity. Even further, porous particles of Al2O3 and other slightly soluble inorganic oxide materials can undergo an increase in abrasivity using these methods.